The present invention relates to lids and, in particular, to lids formed from laminates of adhesive and lid material.
There are many approaches to packaging electronic devices and other goods for protection against such hazards as handling, mechanical damage, environmental exposure, chemical attack, and other potentially adverse elements. As a result of both functional and aesthetic requirements, these devices are typically encased in several levels of packaging. The outermost level is most likely the housing or enclosure of the apparatus in which such devices are employed, such as a computer case, TV receiver enclosure, cellular telephone housing and the like.
Generally, electronics devices at the electrical component level, such as microprocessors and other semiconductor devices, are packaged with a first level of protection in the form of a metal or ceramic package or by a solid organic encapsulation. Besides the traditional xe2x80x9cDual-In-Linexe2x80x9d package in which an electronic component mounted to a lead frame is molded within an epoxy compound, the equivalent may be created by applying a glob of liquid plastic epoxy to form the so-called xe2x80x9cglob-topxe2x80x9d encapsulation. See, e.g., T. Gabrykewioz, et al,. xe2x80x9cGlob-Top Material Selection for Flip Chip Devicesxe2x80x9d, Proceedings of the 1986 International Symposium on Microelectronics, 1986, pages 107 et seq., and Paul Collander, et al., xe2x80x9cHumidity Testing of Plastic Coated Integrated Test Circuitsxe2x80x9d, Proceedings of 1987 International Symposium on Microelectronics, 1987, pages 249 et seq. However, because of substantial differences in the coefficients of thermal expansion between the substrate or device and the encapsulant, substantial peel stress occurs in a coated structure (rather than compressive stress as occurs in a molded structure) which limits the long-term reliability of such devices, as reported by R. Schwartz, xe2x80x9cMicroelectronic Packaging: 11xe2x80x9d, Journal of the American Ceramic Society, 63(4). 1984, pages 577-581. In some cases, an electronic semiconductor device may not be able to be encapsulated by a solid encapsulant because of the adverse effects of stress induced in the device by direct contact with the encapsulant. In other cases, the cost of the encapsulation material and/or its application may be too costly. It is interesting to note that in detailed testing of actual devices, packaged devices that allowed reasonable flow or xe2x80x9cbreathingxe2x80x9d of moisture into and out of the packages proved to be more reliable than those molded devices that sought to exclude such moisture penetration, as reported by E. B. Hakim in the xe2x80x9cUS Army Panama Field Test of Plastic Encapsulated Devicesxe2x80x9d, AD-A048 413, July 1977.
When several packaged electronic devices have been assembled into a functional unit, such as a printed wiring circuit board or other electronic substrate, they are then protected by an exterior lid or cover of the functional unit that forms a housing therefor. These lids or covers may be attached to the functional unit with adhesive, solder, screws or other mechanical fasteners.
Certain electronic devices may need a thermal connection to transfer heat away from the device, which need cannot be satisfied by an encapsulant or molding compound. A thermally-conductive metal lid attached with adhesives that also serve as a thermal interface with the device inside the package or laced with another thermally-conductive media between the lid and the heat generating device have been employed to provide such thermal connection, however, the conventional application of such techniques has been imprecise and so the effectiveness and conductivity of the thermal connection may be uncertain.
Certain other cases may need the lid or cover to be electrically conductive and connected to the electrical ground of the finished device to provide attenuation of electromagnetic interference (EMI) to and from other devices. This requirement cannot be easily met with an insulating organic encapsulant and soldering may be inconvenient or undesirable because of the effect of the soldering temperature on electronic devices that are attached by soldering. In reworking or repairing a device employing soldered covers, de-soldering the cover may also cause damage to or de-soldering of the covered electronic devices.
In fact, most electronic devices used in military, space and other high reliability applications employ a hermetic-seal package to prevent moisture and other contaminants and adverse elements from affecting the electronic devices enclosed thereby. However, hermetic packages are very expensive to implement. In addition, either soldered or brazed metal packages or housings are usually employed in military and space applications requiring an electrically-conductive package or housing for EMI protection. In order to prevent damage to the electronic devices mounted in such hermetic package by the generally high processing temperatures necessary for soldering or brazing, each package has to be selectively heated only in the local area at the package rim to which the lid is soldered or brazed. Thus the processing time and work required to attach the protective lid is high, and therefore costly.
In addition, both the sealing material and the lid material of hermetic devices must be selected from materials having coefficients of thermal expansion (CTE) that substantially match the CTE of the electronic package and of the electronic device mounted therein. This requirement of matching the respective CTE of the board substrate to that of the sealing materials, and to that of the lids also increases the cost of the finished devices. In general, the cost of both the materials meeting these requirements and processing they require are prohibitive for general application commercial electronics.
Lids and covers are used to a certain extent in commercial electronics, for example, where special requirements exist. One such requirement applies to frequency selective electronic devices that are susceptible to stress-induced frequency distortion, such as certain oscillators, crystals, oscillators, acoustic wave filters and other like devices employed in communication equipment. Lids for such devices are generally attached by an adhesive in the form of dispensable paste or die-cut preforms that may be applied to the lid shortly before the lid attachment bonding process. Where the volume of production is high, for example, lids are pre-coated with adhesive or have adhesive preforms pre-applied, and the adhesives employed are those that will flow and cure when heated and applied under pressure during the lid attachment process.
However, the cost of pre-coating adhesives and of pre-application of adhesive preforms onto lids or covers by conventional methods is still quite high. Typically, adhesives in liquidous form are dispensed with a programmable automatic dispenser or are roller-coated onto the sealing areas of each lid. The adhesive is subsequently dried or B-staged at a temperature and for a time substantially less than the curing temperature and time of the adhesive. The liquidous adhesive thus becomes solid state adhesive on each lid either through solvent evaporation or through chemical cross-linking, or both, during what is generically termed as xe2x80x9cB-staging.xe2x80x9d For example, in U.S. Pat. No. 5,667,884 entitled xe2x80x9cArea Bonding Conductive Adhesive Preformsxe2x80x9d issued to J. C. Bolger, Example VII thereof describes stamping and cutting copper strip into individual square domed covers, cutting tape adhesive into individual squares slightly larger than the individual covers, and tacking the individual adhesive squares to the individual copper cover squares. These tacked covers are pre-heated and then joined to a pre-heated semiconductor die attached to a preform in a clamshell heating fixture.
Thus, there is a need for an efficient method of pre-coating and pre-application of adhesive onto lids and covers to provide a cost-effective solution in protecting sensitive devices such as electronic devices. It may also be desirable that the adhesive attaching the lids be removable at a temperature and with a force low enough that neither the electronic substrate nor the electronic elements under the lid be damaged thereby, for example, at a temperature less than the melting temperature of solder.
There is also a need for attachment of EMI-shielding lids or covers at a temperature lower than the melting temperature of solder. This will be even more useful it the adhesive that attaches the lid or cover is electrically conductive and bonds instantly upon reaching a bonding temperature that is substantially below the general melting temperature of solder, or about 220xc2x0 C.
With the advancing of semiconductor technology to produce more powerful microprocessors and other semiconductor components that generate substantial heat, there is also a need for a cost-effective lid attachment that also serves as thermal spreader to facilitate removal of such heat.
To this end, the method of making a plurality of laminated lids of the present invention comprises:
laminating a sheet of lid material and an adhesive; and
stamping the plurality of laminated lids out of the laminated sheet of lid material and adhesive.
According to another aspect of the present invention, a method of contemporaneously making a plurality of lids having an adhesive laminated thereto comprises:
obtaining a sheet of lid material;
depositing a layer of adhesive on the sheet of lid material to form a laminated sheet of lid material and adhesive;
locating the laminated sheet between a pair of tooling plates, one tooling plate having a plurality of recesses therein at positions at which respective ones of the plurality of lids are to be formed, and the other tooling plate having a plurality of projections therefrom in positions corresponding to the recesses in the first tooling plate;
forming the plurality of lids in the laminated sheet by moving the tooling plates together to place the projections of the second tooling plate into the recesses of the first tooling plate, whereby the laminated sheet is formed into the recesses by the projections; and
cutting the plurality of formed lids from the formed laminate sheet.